Secretary of state says officials and US legislators are in talks to save the landmark nuclear agreement with Tehran.

Secretary of State Rex Tillerson says the United States is working on a plan "to fix" the Iran nuclear deal as speculation continues on whether President Donald Trump will scrap the famous agreement.

Changes to American law codifying US participation in the 2015 agreement could come as early as next week as US officials negotiate with legislators, Tillerson told The Associated Press on Friday.

"The president said he is either going to fix it or cancel it," Tillerson was quoted as saying. "We are in the process of trying to deliver on the promise he made to fix it."

Trump faces a series of deadlines this month on how to proceed with the landmark nuclear accord signed by his predecessor, Barack Obama, also involving China, Russia, Germany, the UK, and France. The US president has described the agreement as "the worst deal ever".

It's unclear what Trump's decision on Iran will be by January 12 when he must decide again whether to certify Tehran's compliance with the nuclear pact - as he's required to do every three months.

In October, Trump declined to certify Iran's compliance, saying sanctions relief was disproportionate to Iran's concessions, adding the agreement goes against US interests.

However, Trump waived the sanctions for another three months and left the decision on staying in or quitting the deal for later, while urging Congress to change the US law concerning the certification.

Tillerson said the administration was speaking with congressional leaders "on a very active basis" about a fix.

He suggested Trump might be inclined to preserve the deal by waiving sanctions again on January 12 if there were signs Congress would act in short order.

"I don't want to suggest we're across the finish line on anything yet," Tillerson said.

One option legislators are discussing with the White House is removing the requirement that Trump certify Iranian compliance. Another possibility is changing the law so certification occurs far less often, officials told AP.

Analyst Bassima Alghussein from the Truman National Security Project said new legislation would act as political cover for Trump.

"What we've seen is really he's unable to come up with a better deal. So what they're trying to do politically is just prevent the president from being the one to have to sign off on it [every three months]," she said.

"What he's done is punted it to Congress and they're looking at ways to be creative now … without essentially scrapping the deal all together."